VIDEO: Fall Out Boy's Emergency Sing-A-Long

After a paperwork problem nixed Fall Out Boy's plans for a surprise free concert in a New York City park, the group vowed that the show would go on – even without instruments.

The band, whose album Folie A Deux will be released Tuesday, led the crowd that had gathered to see them at Washington Square Park in an a cappella sing-a-long Monday. The moment was captured on amateur video and by bassist Pete Wentz's own camera phone, with a picture and apology posted on his blog.

"I genuinely feel sorry that we weren't able to play today," Wentz wrote to fans. "They said if we picked up a guitar we would be sorry ... But we knew this probably was gonna happen. We didn't go through the proper process 'cause we wanted it to be free and fun and spontaneous. So it was our fault."

"Anyway," he adds, "thanks for singing with us. We'll make it up to you. It means a lot that you even showed up."